Size limit hook doesn't reject pushes where an oversized file was added then deleted

To reproduce:

• Set up a pre-receive hook in BB that limits the size of pushed files.
• Add a file larger than the size limit to your local repo and commit.
• git rm the file and commit again.
• Push to BB.

Expected behaviour: Push is rejected because the large file is still present in history.

Actual behaviour: Push is allowed.

(NB. In the case where the file was added prior to the push, and the push is just deleting it, then the hook should allow the push - and indeed it does as per SRBITB-251.)